1. Field of the Invention
The present invention relates to a lithographic apparatus, a method of manufacturing a device, and a substrate holder.
2. Brief Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. In general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference thereto.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in patents U.S. Pat. No. 5,969,441 and U.S. Pat. No. 6,262,796, which are incorporated herein by reference thereto.
To hold the substrate to the substrate table, a so-called burl plate may be used. A burl plate described in patent U.S. Pat. No. 6,232,615 (which document is incorporated herein by reference thereto) comprises a plate with a matrix arrangement of protrusions, or burls, on one face and a wall surrounding the matrix of burls. The burls all have a height of 150 μm. Holes in the burl plate lead to a vacuum system whereby the space below the wafer can be evacuated. The pressure differential between the normal atmospheric pressure above the substrate and the evacuated region below clamps the substrate firmly to the burl plate. The vacuum ports are relatively numerous, e.g. 20 or more, and are disposed in two concentric rings.
Other known designs of substrate holder have a relatively small number of vacuum ports, e.g. 3 or 4. For example, U.S. Pat. No. 5,923,408 discloses a substrate holder with three vacuum ports and protrusions that have total height of not less than 550 μm—made up of a narrow section of diameter 100 μm and height 50 μm on top of a wider section of diameter not less than 1 mm and a height not less than 500 μm. U.S. Pat. No. 5,324,012 discloses a pin chuck-type holder with a single vacuum port. The pin-type protrusions are said to have a height of from 10 μm to 500 μm but no specific examples are given. EP 1 077 393 A2 describes substrate holders that have one, four or eight vacuum ports and various arrangements of pin-like protrusions, but does not disclose the height of the pins. EP 0 803 904 A2 discloses a substrate holder that has pins of a height between 17.8–30.5 μm and four vacuum ports in a central region. GB 2 149 697 A describes a vacuum chuck with a plurality of pin-type protrusions of 50 μm in height and six vacuum ports.
The known designs of substrate holder suffer from the problem that if a concave (dished) substrate is placed on them it fails to be clamped because the wide gap between the raised edges of the substrate and the surrounding wall means that no vacuum develops underneath the substrate. Substrates can become concave due to processes carried out on them to build devices and may be discarded if they become too dished to be clamped onto the substrate table. The need to discard such substrates reduces yield and throughput.